How to calculate exactly how many tiles you need

There are two ways a tile order goes wrong, and they are equally expensive in different ways. The first is ordering too little — you run out mid-install, the supplier no longer has stock from your dye lot, and the replacement tile is visibly different in colour. The second is ordering too much — tile is heavy, expensive and difficult to return. Getting the order right at the start is the cheapest and most stressful-saving decision you will make in the entire project.

Step 1: measure each surface separately

The first rule is simple: never aggregate. Each surface gets its own measurement, recorded separately, and only summed at the end. Different surfaces may use different tiles, different layouts and different wastage percentages, and aggregating early hides those differences.

For floors, multiply length by width to get the floor area in square metres. A 3.0 × 2.5m bathroom is 7.5m². For walls, measure each wall as height × width, recorded individually. Be specific about which surfaces are being tiled — floor only, floor plus a splashback at vanity height, floor plus walls to 1200mm, full floor-to-ceiling on shower walls. The measurement schedule should reflect the actual install spec.

Keep the measurements written down in a single document with the room name, the surface description and the measurement. This document becomes the basis of your order.

Step 2: subtract permanent fixtures

Once each surface is measured, decide what to subtract. The principle is to subtract only those areas that are genuinely not being tiled, and when in doubt to leave the area in and treat the extra tile as part of your wastage pool.

Subtract a doorway opening from a wall measurement — there is no tile in the doorway. Subtract a shower base if it is a separate prefabricated unit rather than a tiled-in floor. Do not subtract a window from a wall measurement, because the reveal around the window will almost always be tiled and the difference is small enough that the labour of calculating it precisely is not worth the saving. Do not subtract a vanity, because the wall behind it is often tiled even if it is hidden. If a fixture is less than half a square metre, leaving it in your calculation buys you a few extra tiles for free, which is exactly what you want.

Step 3: convert to m² and add wastage %

With each surface measured and relevant fixtures subtracted, sum the surfaces to get a total net area to be tiled. To this you add a wastage percentage, which covers cuts, breakages, tiles rejected on opening the box, and spare tiles for future repairs.

Add additional wastage for two situations. Very small rooms generate proportionally more cuts — a 2m² powder room should carry an extra 2–3% on top of the layout figure. Future-repair stock is the second adjustment — at least one full box of every tile, kept labelled in dry storage, against the day a tile cracks or a future renovation extends into the same room.

Step 4: translate to boxes

Tile is sold by the box, not by the square metre, and box coverage varies dramatically by tile size. A box of 600×1200 porcelain typically covers around 1.44m². A box of 100×200 subway tile may cover only 0.6m². A box of 1200×2400 large-format porcelain may cover up to 2.88m² but contain only one tile.

Always calculate in m² first and convert to boxes only as the final step. Take the wastage-adjusted total, divide by the box coverage, and round up to the nearest full box. Never round down. Confirm two pieces of information at the supplier: the actual coverage per box, and the number of tiles per box. Most suppliers in Australia sell by full box only.

Step 5: order from one batch

Tile is fired in batches, and colour varies subtly between batches even within the same product line. The batch number — sometimes called the dye lot — is printed on every box. Tiles from one batch are colour-consistent; tiles from two batches are not, and the difference is visible on a wall regardless of how careful the install.

When placing the order, ask the supplier explicitly to fill the entire order from a single batch. If the order spans multiple batches, discuss where the batch break should fall — use the second batch on a less prominent surface such as a wardrobe floor rather than the front wall of the shower. If you run short mid-installation and need to top up, ask for a batch match first. The defensive position is to order enough the first time.

Worked example: a 3×3m bathroom

Take a square 3.0 by 3.0 metre bathroom, 2.4 metres high, fully tiled — floor and walls floor to ceiling — in a straight-lay 600×1200 porcelain. The shower occupies one corner with a frameless screen; there is one doorway and one window.

This calculation takes ten minutes with a tape measure, a notepad and a calculator. It is the difference between a renovation that runs smoothly and one that stalls waiting for a second shipment.

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